Stable tin free catalysts for electroless metallization

ABSTRACT

Catalysts which include nanoparticles of palladium metal and cellulose derivatives are used in electroless metal plating. The palladium catalysts are free of tin.

This application claims the benefit of priority under 35 U.S.C. §119(e)to U.S. Provisional Application No. 61/524,416, filed Aug. 17, 2011, theentire contents of which application are incorporated herein byreference.

The present invention is directed to stable aqueous solutions of tinfree palladium catalysts for electroless metallization. Morespecifically, the present invention is directed to stable aqueoussolutions of tin free palladium catalysts for electroless metallizationwhere the catalysts form nanoparticles of palladium metal and celluloseor cellulose derivatives.

Electroless metal deposition is a well-known process for depositingmetallic layers on substrate surfaces. Electroless plating of adielectric surface requires the prior application of a catalyst. Themost commonly used method of catalyzing or activating dielectrics, suchas non-conductive sections of laminated substrates used in themanufacture of printed circuit boards, is to treat the substrate with anaqueous tin/palladium colloid in an acidic chloride medium. Thestructure of the colloid has been extensively studied. In general, thecolloid includes a palladium metal core surrounded by a stabilizinglayer of tin(II) ions, essentially a shell of SnCl₃ ⁻ complexes whichact as surface stabilizing groups to avoid agglomeration of the colloidsin suspension.

In the activation process the tin/palladium colloid catalyst is adsorbedonto a dielectric substrate, such as epoxy or polyimide containingsubstrate, to activate electroless metal deposition. Theoretically thecatalyst functions as a carrier in the path of electron transfer fromreducing agents to metal ions in the electroless metal plating bath.Although performance of electroless plating is influenced by manyfactors, such as additive composition of the plating solution, theactivation step is key for controlling the rate and mechanism ofelectroless plating.

In recent years, along with the reduction in size and desired increasein the performance of electronic devices, the demand for defect freeelectronic circuits in the electronic packaging industry has becomehigher. Although the tin/palladium colloid has been commercially used asan activator for electroless metal plating for decades and has givenacceptable service, it has many disadvantages which are becoming morepronounced as the demand for higher quality electronic devicesincreases. The stability of the tin/palladium colloid is a majorconcern. As mentioned above the tin/palladium colloid is stabilized by alayer of tin(II) ions and its counter anions can prevent palladium fromagglomerating. The catalyst is sensitive to air and readily oxidizes totin(IV), thus the colloid cannot maintain its colloidal structure. Thisoxidation is further promoted by increase in temperature and agitationduring electroless plating. If the concentration of tin(II) falls tocritical levels, such as close to zero, palladium metal particles growin size, agglomerate and precipitate, thus becoming catalyticallyinactive. As a result there is an increase in demand for a more stablecatalyst. In addition the high and fluctuating cost of palladium hasencouraged the industry to search for a less costly metal.

Considerable efforts have been made to find new and improved catalysts.Because of the high cost of palladium, much effort has been directedtoward development of palladium free catalysts, such as colloidal silvercatalysts. Another direction that research has taken is towards a tinfree palladium catalyst since stannous chloride is costly and theoxidized tin requires a separate acceleration step. The accelerationstep is an extra step in the metallization process and it often stripsoff some catalyst on substrates, especially on substrates of glassfiber, causing voids on the plated substrate surface. However, such tinfree catalysts have shown to be insufficiently active and reliable forthrough-hole plating in printed circuit board manufacture. Further, suchcatalysts typically become progressively less active upon storage, thusrendering such catalyst unreliable and impractical for commercial use.

Alternative stabilizing moieties for tin complexes, such aspolyvinylpyrrolidone (PVP) and dendrimers, have been investigated.Stable and uniform PVP protected nanoparticles have been reported byvarious research groups in the literature. Other metal colloids, such assilver/palladium and copper/palladium in which palladium is partiallyreplaced by less expensive metals have also been reported in theliterature; however, such alternative catalysts have not beencommercially acceptable. Ionic palladium variants have been usedcommercially, but they require an extra reducing step. Accordingly,there is still a need for a stable and reliable electroless metalplating catalyst.

Aqueous catalyst solutions include one or more antioxidant,nanoparticles of palladium metal and one or more compounds chosen frompolymers having a formula:

wherein R₁, R₂, R₃, R₄, R₅, R₆ and R₇ are the same or different and arechosen from —H, —CH₂COOX, —C(O)—CH₃, —C(O)—(CH₂)_(z)—CH₃ and

wherein n is an integer of at least 2, z is an integer of at least 1 andX is —H or a counter cation, and polymers of a reaction product of apolymer having a formula:

wherein R₉, R₁₀, R₁₁, R₁₂, R₁₃, R₁₄, and R₁₅ are the same or differentand are chosen from —H, —CH₃, —CH₂CH₃, —CH₂OH, —[CH₂CHR₈]_(x)—OH,—CH₂CH(OH)CH₃ and —(CH₂CHR₈O)_(y)—H, with the proviso that at least oneof R₉, R₁₀, R₁₁, R₁₂, R₁₃, R₁₄ and R₁₅ is —CH₂OH, —[CH₂CHR₈]_(x)—OH,—CH₂CH(OH)CH₃ or —(CH₂CHR₈O)_(y)—H, wherein R₈ is —H or —CH₃, x and yare integers of at least 1 and n and z are as described above, and aquaternary compound having a formula:

wherein m is an integer from 1 to 16, Y is halogen, Z⁻ is a counteranion, R₁₆, R₁₇ and R₁₈ are the same or different and are —H, —CH₃ or—(CH₂)_(p)—CH₃, and R₁₉ is —H or —CH₃ and p is an integer of 1 to 9, andone or more cross-linking agents; and the catalyst is free of tin.

Methods include:

-   -   a) providing a substrate;    -   b) applying an aqueous catalyst solution to the substrate, the        aqueous catalyst solution includes one or more antioxidants,        nanoparticles of palladium metal and one or more compounds        chosen from polymers having a formula:

wherein R₁, R₂, R₃, R₄, R₅, R₆ and R₇ are the same or different and arechosen from —H, —CH₂COOX, —C(O)—CH₃, —C(O)—(CH₂)_(z)—CH₃ and

wherein n is an integer of at least 2, z is an integer of at least 1 andX is —H or a counter cation, and a reaction product of a polymer havinga formula:

wherein R₉, R₁₀, R₁₁, R₁₂, R₁₃, R₁₄, and R₁₅ are the same or differentand are chosen from —H, —CH₃, —CH₂CH₃, —CH₂OH, —[CH₂CHR₈]_(x)—OH,—CH₂CH(OH)CH₃ and —(CH₂CHR₈O)_(y)—H, with the proviso that at least oneof R₉, R₁₀, R₁₁, R₁₂, R₁₃, R₁₄, and R₁₅ is —CH₂OH, —[CH₂CHR₈]_(x)—OH,—CH₂CH(OH)CH₃, —(CH₂CHR₈O)_(y)—H wherein R₈ is —H or —CH₃, x and y areintegers of at least 1 and n and z are as described above, and aquaternary compound having a formula:

wherein m is an integer from 1 to 16, Y is halogen, Z⁻is a counteranion, R₁₆, R₁₇ and R₁₈ are the same or different and are —H, —CH₃ or—(CH₂)_(p)—CH₃, and R₁₉ is —H or —CH₃ and p is an integer of 1 to 9, andone or more cross-linking agents; and the catalyst is free of tin; and

-   -   c) electrolessly depositing metal onto the substrate using an        electroless metal plating bath.

The catalysts may be used to electrolessly plate metals on substrates,including substrates of dielectric materials and are stable upon storageas well as during electroless metal plating since they do not readilyoxidize as compared to conventional tin/palladium catalysts. Thecellulose stabilizers function as do stannous chloride in conventionaltin/palladium catalysts except that the cellulose stabilizers arebiodegradable, thus they do not present an environmental hazard as doesstannous chloride upon disposal. The cellulose stabilizers are availablein large quantities with a fraction of the cost of stannous chloride.The raw materials used to make the stabilizers are readily availablefrom plant life which is essentially ubiquitous. The cellulosestabilized palladium catalysts enable electroless metal plating withoutan acceleration step and enable good metal coverage of the substrate,even walls of through-holes of printed circuit boards.

As used throughout this specification, the abbreviations given belowhave the following meanings, unless the context clearly indicatesotherwise: g=gram; mg=milligram; ml=milliliter; L=liter; cm=centimeter;m=meter; mm=millimeter; μm=micron; nm=nanometers; ppm=parts per million;° C.=degrees Centigrade; g/L=grams per liter; DI=deionized; wt %=percentby weight; and T_(g)=glass transition temperature.

The terms “printed circuit board” and “printed wiring board” are usedinterchangeably throughout this specification. The terms “plating” and“deposition” are used interchangeably throughout this specification. Allamounts are percent by weight, unless otherwise noted. All numericalranges are inclusive and combinable in any order except where it islogical that such numerical ranges are constrained to add up to 100%.

Aqueous catalyst solutions include nanoparticles of palladium metal andone or more stabilizing polymers having a formula:

wherein R₁, R₂, R₃, R₄, R₅, R₆ and R₇ are the same or different and arechosen from —H, —CH₂COOX, —C(O)—CH₃, —C(O)—(CH₂)_(z)—CH₃ and

wherein n is an integer of at least 2, typically from 2 to 20,preferably from 2 to 15, more preferably from 5 to 10, z is an integerof at least 1, typically from 1 to 10, preferably from 2 to 5, and X is—H or a counter cation, such as sodium, potassium, ammonium ion or analkaline earth metal, typically sodium or potassium. Preferably, atleast one of R₁, R₂, R₃, R₄, R₅, R₆ and R₇ is, —C(O)—CH₃ or —CH₂COOX,more preferably, at least one of R₂, R₃, R₅, R₆ and R₇ is —CH₂COOX, andpreferably when R₁, R₂, R₃, R₄, R₅, R₆ or R₇ is not —C(O)—CH₃ or—CH₂COOX, it is —H. Exemplary polymers are carboxymethyl cellulose andcellulose acetate. The catalyst is tin free.

The stabilizing polymers can also be a reaction product of a polymerhaving a formula:

wherein R₉, R₁₀, R₁₁, R₁₂, R₁₃, R₁₄, and R₁₅ are the same or differentand are chosen from —H, —CH₃, —CH₂CH₃, —CH₂OH, —[CH₂CHR₈]_(x)—OH,—CH₂CH(OH)CH₃ and —(CH₂CHR₈O)_(y)—H, with the proviso that at least oneof R₉, R₁₀, R₁₁, R₁₂, R₁₃, R₁₄ and R₁₅ is —CH₂OH, —[CH₂CHR₈]_(x)—OH,—CH₂CH(OH)CH₃ or —(CH₂CHR₈O)_(y)—H, preferably, at least one of R₁₀,R₁₁, R₁₃, R₁₄ and R₁₅ is —[CH₂CHR₈]_(x)—OH or —(CH₂CHR₈O)_(y)—H, whereinR₈ is —H or —CH₃, x and y are integers of at least 1, typically from 1to 10, preferably from 2 to 5 and n and z are as described above, and aquaternary compound having a formula:

wherein m is an integer from 1 to 16, Y is halogen, such as fluorine,chlorine, bromine or iodine, preferably the halogen is chlorine, Z⁻ is acounter anion, such as a halide such as fluoride, chloride, bromide, oriodide, preferably chloride, nitrate, nitrite, phosphate, hydroxide orcarboxylate, such as acetate or propionate, R₁₆, R₁₇ and R₁₈ are thesame or different and are —H, —CH₃ or —(CH₂)_(p)—CH₃, and R₁₉ is —H or—CH₃ and p is an integer of 1 to 9, and one or more cross-linkingagents. The catalyst is tin free. Exemplary quaternary compounds areglycidyl trimethylammonium chloride,2,3-epoxypropyl-N,N,N-trimethylammonium chloride,3-chloro-2-hydroxypropyl-N,N,N-trimethylammonium chloride,3-chloro-2-hydroxypropyle-N,N,N-dimethylethanolammonium chloride and1,3-bis-(3-chloro-2-hydroxypropyl-N,N-dimethylammonium)-N-propanedichloride. A preferred reaction product of the cellulose derivative andquaternary ammonium compound is the reaction product of hydroxyethylcellulose and glycidyl trimethylammonium chloride.

Examples of cross-linking agents include, but are not limited to,formaldehyde, methylolated nitrogen compounds, such as dimethylolurea,dimethylolethyleneurea and dimethylolimidazolidone; dicarboxylic acids,such as maleic acid; dialdehydes, such as glyoxal; diepoxides, such as1,2,3,4-diepoxybutane and 1,2,5,6-diepoxyhexane; diisocyantes; divinylcompounds, such as divinylsulfone; dihalogen compounds, such asdichloroacetone, dichloroacetic acid, 1,3-dichloropropan-2-ol,dichloroethane, 2,3-dibromo-1-propanol, 2,3-dichloro-1-propanol and2,2-dichloroethyl ether; halohydrins, such as epichlorohydrine;bis(epoxypropyl)ether; vinylcyclohexenedioxide; ethyleneglycol-bis(epoxypropyl)ether; vinylcyclohexenedioxide; ethyleneglycol-bis(epoxypropyl)ether; 1,3-bis(β-hyroxy-Λ-chloropropoxy)ether;1,3-bis(β-hydroxy-Λ-chloropropoxy)ethane; methylenebis(acrylamide);N,N′-dimethylol(methylenebis(acrylamide)); triacrylolhexahydrotriazine;acrylamidomethylene chloroacetamide; 2,4,6-trichloropyrimidine;2,4,5,6-tetrachloropyrimidine; cyanuric chloride; triallylcyanuratephosphorusoxychloride; bis(acrylamido)acetic acid; di-epoxy compoundsand haloepxoy compounds, such as1,3-bis(glycidyldimethylammonium)propanedichloride and epichlorohydrin.The preferred cross-linking agents are the di-epoxy and haloepoxycompounds.

The polymers may be made by known methods in the art and literature andmany are commercially available. An example of commercially availablesodium carboxymethyl cellulose is AQUALON by Ashland and a commerciallyavailable polymer of hydroxyethyl cellulose and glycidyltrimethylammonium chloride is UCARE JR-125 available from AmercholCorporation. Methods for making the cellulose and cellulose derivativesand quaternary ammonium compound polymers are disclosed in U.S. Pat. No.5,780,616.

The weight average molecular weights of the polymers may vary. Typicallythey range from 10,000 and greater, more typically from 10,000 to300,000.

The stabilizing polymers are included in the aqueous catalyst solutionsin sufficient amounts to provide nanoparticle stabilization. Mixtures ofthe various stabilizing polymers described above may be included in theaqueous catalysts. Minor experimentation may be required to determinethe amount of a particular stabilizer or combination of stabilizers tostabilize a given metal nanoparticle. In general, one or morestabilizing polymers are included in the aqueous catalyst solutions inamounts of 10 mg/L to 10 g/L, preferably from 20 mg/L to 1 g/L.

One or more antioxidants are included in the aqueous catalyst solutions.Conventional antioxidants may be included and may be included inconventional amounts. Typically antioxidants are included in amounts of0.1 g/l to 10 g/l, preferably from 0.2 g/L to 5 g/L. Such antioxidantsinclude, but are not limited to, ascorbic acid, phenolic acid,polyphenolic compounds, such as but not limited to, hydroxybenzoic acidand derivatives, gallic acid, hydroxybenzoaldehydes, catechol,hydroquinone, catechin and flavonoids.

One or more reducing agents are included to reduce palladium ions topalladium metal. Conventional reducing agents known to reduce palladiumions to palladium metal may be used. Such reducing agents include, butare not limited to, dimethylamine borane, sodium borohydride, ascorbicacid, iso-ascorbic acid, sodium hypophosphite, hydrazine hydrate, formicacid and formaldehyde. Reducing agents are included in amounts to reducesubstantially all of the palladium ions to palladium metal. Such amountsare generally conventional amounts and are well known by those of skillin the art.

Sources of palladium metal include any water soluble palladium salt.Such salts are included to provide palladium metal in amounts of 10 ppmto 5000 ppm, preferably from 300 ppm to 1500 ppm. Water solublepalladium salts include, but are not limited to, palladium sodiumchloride, palladium chloride, palladium acetate, palladium potassiumchloride and palladium nitrate.

The components which make up the aqueous catalyst may be combined in anyorder. Any suitable method known in the art and literature may be usedto prepare the aqueous catalyst solution. While the specific parametersand amounts of components may vary from one method to the other, ingeneral, one or more of the stabilizing polymers is first solubilized ina sufficient amount of water. One or more sources of palladium metal asan aqueous solution is combined with the stabilizer solution withvigorous agitation to form a uniform mixture. An aqueous solutioncontaining one or more reducing agents is then mixed with the mixture ofstabilizers and palladium salts with vigorous agitation to reduce thepalladium ions to palladium metal. The process steps and solution aretypically done at room temperature; however, temperatures may be variedto assist in solubilizing reaction components and to encourage reductionof palladium ions to palladium metal. While not being bound by theory,the stabilizers may coat or surround portions or most of the palladiumparticles to stabilize the catalyst solution. The particles of palladiummetal and stabilizer range in size from 1 nm to 1000 nm or such as from2 nm to 500 nm Preferably the particles range in size from 2 nm to 300nm, more preferably from 2 nm to 100 nm, most preferably from 2 nm to 10nm.

One or more acids may be added to the catalyst to provide a pH range ofless than 7, preferably from 1-6.5, more preferably from 2-6. Inorganicor organic acids may be used in sufficient amounts to maintain the pH atthe desired range. Mixtures of inorganic and organic acids also may beused. Examples of inorganic acids are hydrochloric acid, sulfuric acidand nitric acid. Organic acids include mono- and polycarboxylic acids,such as dicarboxylic acids. Examples of organic acids are benzoic acidand its derivatives, such as hydroxybenzoic acid, ascorbic acid,iso-ascorbic acid, malic acid, maleic acid, gallic acid, acetic acid,citric acid and tartaric acid.

The catalysts may be used to electrolessly metal plate varioussubstrates which are known to be capable of being electrolessly metalplated. Substrates include, but are not limited to, materials includinginorganic and organic substances such as glass, ceramics, porcelain,resins, paper, cloth and combinations thereof. Metal-clad and uncladmaterials also are substrates which may be metal plated using thecatalyst.

Substrates also include printed circuit boards. Such printed circuitboards include metal-clad and unclad with thermosetting resins,thermoplastic resins and combinations thereof, including fiber, such asfiberglass, and impregnated embodiments of the foregoing.

Thermoplastic resins include, but are not limited to, acetal resins,acrylics, such as methyl acrylate, cellulosic resins, such as ethylacetate, cellulose propionate, cellulose acetate butyrate and cellulosenitrate, polyethers, nylon, polyethylene, polystyrene, styrene blends,such as acrylonitrile styrene and copolymers and acrylonitrile-butadienestyrene copolymers, polycarbonates, polychlorotrifluoroethylene, andvinylpolymers and copolymers, such as vinyl acetate, vinyl alcohol,vinyl butyral, vinyl chloride, vinyl chloride-acetate copolymer,vinylidene chloride and vinyl formal.

Thermosetting resins include, but are not limited to, allyl phthalate,furane, melamine-formaldehyde, phenol-formaldehyde and phenol-furfuralcopolymers, alone or compounded with butadiene acrylonitrile copolymersor acrylonitrile-butadiene-styrene copolymers, polyacrylic esters,silicones, urea formaldehydes, epoxy resins, allyl resins, glycerylphthalates and polyesters.

Porous materials include, but are not limited to paper, wood,fiberglass, cloth and fibers, such as natural and synthetic fibers, suchas cotton fibers and polyester fibers.

The catalysts may be used to plate both low and high T_(g) resins. LowT_(g) resins have a T_(g) below 160° C. and high T_(g) resins have aT_(g) of 160° C. and above. Typically high T_(g) resins have a T_(g) of160° C. to 280° C. or such as from 170° C. to 240° C. High T_(g) polymerresins include, but are not limited to, polytetrafluoroethylene (PTFE)and polytetrafluoroethylene blends. Such blends include, for example,PTFE with polypheneylene oxides and cyanate esters. Other classes ofpolymer resins which include resins with a high Tg include, but are notlimited to, epoxy resins, such as difunctional and multifunctional epoxyresins, bimaleimide/triazine and epoxy resins (BT epoxy),epoxy/polyphenylene oxide resins, acrylonitrile butadienestyrene,polycarbonates (PC), polyphenylene oxides (PPO), polypheneylene ethers(PPE), polyphenylene sulfides (PPS), polysulfones (PS), polyamides,polyesters such as polyethyleneterephthalate (PET) andpolybutyleneterephthalate (PBT), polyetherketones (PEEK), liquid crystalpolymers, polyurethanes, polyetherimides, epoxies and compositesthereof.

The catalyst may be used to deposit metals on the walls of through-holesor vias of printed circuit boards. The catalysts may be used in bothhorizontal and vertical processes of manufacturing printed circuitboards.

The aqueous catalysts may be used with conventional electroless metalplating baths. While it is envisioned that the catalysts may be used toelectrolessly deposit any metal which may be electrolessly plated,typically, the metal is chosen from copper, copper alloys, nickel ornickel alloys. More typically the metal is chosen from copper and copperalloys, most typically copper is used.

Conventional electroless copper or copper alloy baths may be used.Typically sources of copper ions include, but are not limited to, watersoluble halides, nitrates, acetates, sulfates and other organic andinorganic salts of copper. Mixtures of one or more of such copper saltsmay be used to provide copper ions. Examples include copper sulfate,such as copper sulfate pentahydrate, copper chloride, copper nitrate,copper hydroxide and copper sulfamate. Conventional amounts of coppersalts may be used in the compositions. Copper ion concentrations in thecomposition may range from 0.5 g/L to 30 g/L or such as from 1 g/L to 20g/L or such as from 5 g/L to 10 g/L.

One or more alloying metals also may be included in the electrolesscompositions. Such alloying metals include, but are not limited to,nickel and tin. Examples of copper alloys include copper/nickel andcopper/tin. Typically the copper alloy is copper/nickel.

Sources of nickel ions for nickel and nickel alloy electroless baths mayinclude one or more conventional water soluble salts of nickel. Sourcesof nickel ions include, but are not limited to, nickel sulfates andnickel halides. Sources of nickel ions may be included in theelectroless alloying compositions in conventional amounts. Typicallysources of nickel ions are included in amounts of 0.5 g/L to 10 g/L orsuch as from 1 g/1 to 5 g/L.

The method steps used in metalizing a substrate may vary depending onwhether the surface to be plated is metal or dielectric. Conventionalsteps used for electrolessly metal plating a substrate may be used withthe catalysts; however, the aqueous polymer stabilized palladiumcatalysts do not require an acceleration step as in many conventionalelectroless plating processes. Accordingly, acceleration steps arepreferably excluded when using the catalyst. In general, the catalyst isapplied to the surface of the substrate to be electrolessly plated witha metal followed by application of the metal plating bath. Electrolessmetal plating parameters, such as temperature and time may beconventional. Conventional substrate preparation methods, such ascleaning or degreasing the substrate surface, roughening ormicro-roughening the surface, etching or micro-etching the surface,solvent swell applications, desmearing through-holes and various rinseand anti-tarnish treatments may be used. Such methods and formulationsare well known in the art and disclosed in the literature.

In general, when the substrate to be metal plated is a dielectricmaterial such as on the surface of a printed circuit board or on thewalls of through-holes, the boards are rinsed with water and cleaned anddegreased followed by desmearing the through-hole walls. Typicallyprepping or softening the dielectric surface or desmearing of thethrough-holes begins with application of a solvent swell.

Any conventional solvent swell may be used. The specific type may varydepending on the type of dielectric material. Examples of dielectricsare disclosed above. Minor experimentation may be done to determinewhich solvent swell is suitable for a particular dielectric material.The T_(g) of the dielectric often determines the type of solvent swellto be used. Solvent swells include, but are not limited to, glycolethers and their associated ether acetates. Conventional amounts ofglycol ethers and their associated ether acetates may be used. Examplesof commercially available solvent swells are CIRCUPOSIT CONDITIONER™3302, CIRCUPOSIT HOLE PREP™ 3303 and CIRCUPOSIT HOLE PREP™ 4120(obtainable from Rohm and Haas Electronic Materials, Marlborough,Mass.).

Optionally, the substrate and through-holes are rinsed with water. Apromoter is then applied. Conventional promoters may be used. Suchpromoters include sulfuric acid, chromic acid, alkaline permanganate orplasma etching. Typically alkaline permanganate is used as the promoter.An example of a commercially available promoter is CIRCUPOSIT PROMOTER™4130 available from Rohm and Haas Electronic Materials, Marlborough,Mass.

Optionally, the substrate and through-holes are rinsed again with water.A neutralizer is then applied to neutralize any residues left by thepromoter. Conventional neutralizers may be used. Typically theneutralizer is an aqueous alkaline solution containing one or moreamines or a solution of 3 wt % peroxide and 3 wt % sulfuric acid.Optionally, the substrate and through-holes are rinsed with water andthen dried.

After the solvent swelling and desmearing an acid or alkalineconditioner may be applied. Conventional conditioners may be used. Suchconditioners may include one or more cationic surfactants, non-ionicsurfactants, complexing agents and pH adjusters or buffers. Examples ofcommercially available acid conditioners are CIRCUPOSIT CONDITIONER™3320 and CIRCUPOSIT CONDITIONER™ 3327 available from Rohm and HaasElectronic Materials, Marlborough, Mass. Suitable alkaline conditionersinclude, but are not limited to, aqueous alkaline surfactant solutionscontaining one or more quaternary amines and polyamines. Examples ofcommercially available alkaline surfactants are CIRCUPOSIT CONDITIONER™231, 3325, 813 and 860 available from Rohm and Haas ElectronicMaterials. Optionally, the substrate and through-holes are rinsed withwater.

Conditioning may be followed by micro-etching. Conventionalmicro-etching compositions may be used. Micro-etching is designed toprovide a micro-roughened metal surface on exposed metal (e.g.innerlayers and surface etch) to enhance subsequent adhesion ofdeposited electroless and later electroplate. Micro-etches include, butare not limited to, 60 g/L to 120 g/L sodium persulfate or sodium orpotassium oxymonopersulfate and sulfuric acid (2%) mixture, or genericsulfuric acid/hydrogen peroxide. An example of a commercially availablemicro-etching composition is CIRCUPOSIT MICROETCH™ 3330 available fromRohm and Haas Electronic Materials. Optionally, the substrate is rinsedwith water.

Optionally a pre-dip is then applied to the micro-etched substrate andthrough-holes. Conventional pre-dip aqueous solutions of inorganic ororganic acids with a pH range typically from 3-5 may be used. An exampleof an inorganic acid solution is 2% to 5% hydrochloric acid. Optionally,the substrate is rinsed with cold water.

A stabilized nanoparticle catalyst is then applied to the substrate andthrough-holes. The substrate and through-holes optionally may be rinsedwith water after application of the catalyst.

The substrate and walls of the through-holes are then plated with metal,such as copper, copper alloy, nickel or nickel alloy with an electrolessbath. Typically copper is plated on the walls of the through-holes.Plating times and temperatures may be conventional. Typically metaldeposition is done at temperatures of 20° C. to 80°, more typically from30° C. to 60° C. The substrate may be immersed in the electrolessplating bath or the electroless may be sprayed onto the substrate.Typically, deposition may be done for 5 seconds to 30 minutes; however,plating times may vary depending on the thickness of the metal on thesubstrate.

Optionally anti-tarnish may be applied to the metal. Conventionalanti-tarnish compositions may be used. An example of a commerciallyavailable anti-tarnish is ANTI TARNISH™ 7130 (available from Rohm andHaas Electronic Materials). The substrate may optionally be rinsed andthen the boards may be dried.

Further processing may include conventional processing by photoimagingand further metal deposition on the substrates such as electrolyticmetal deposition of, for example, copper, copper alloys, tin and tinalloys.

The catalysts may be used to electrolessly plate metals on substrates,including substrates of dielectric materials and are stable upon storageas well as during electroless metal plating since they do not readilyoxidize as compared to conventional tin/palladium catalysts. Thecellulose stabilizers function as do stannous chloride in conventionaltin/palladium catalysts except that the cellulose stabilizers arebiodegradable, thus they do not present an environmental hazard as doesstannous chloride upon disposal. The cellulose stabilizers are availablein large quantities with a fraction of the cost of stannous chloride.The raw materials used to make the stabilizers are readily availablefrom plant life which is essentially ubiquitous. The cellulosestabilized palladium catalysts enable electroless metal plating withoutan acceleration step and enable good metal coverage of the substrate,even walls of through-holes of printed circuit boards.

The following examples are not intended to limit the scope of theinvention but are intended to further illustrate it.

EXAMPLE 1

Carboxymethyl cellulose/palladium catalysts were prepared by dissolving40 mg carboxymethyl cellulose sodium salt in a beaker containing 250 mlDI water at room temperature. With stirring, 172 mg Na₂PdCl₄ in 10 ml DIwater was added and the mixture was vigorously mixed. 250 mg NaBH₄ in 10ml DI water was added to the solution mixture with very strongagitation. The solution quickly changed from yellow to black, indicatingreduction of palladium ions to palladium metal. The average particlesize of the palladium metal was 5 nm. The particles were measured with atransmission electron microscope. The solution of the as-synthesizedcatalyst had a pH between 8 and 9 as measured using an ACCUMET AB15 pHmeter from Fisher Scientific. The beaker containing the aqueous catalystsolution was placed in a 50° C. water bath for 12 hours to test itsstability. After 12 hours the solution was observed and there was noobservable precipitate indicating that the catalyst was still stable.

The catalyst solution was used as a stock solution and 8 aliquots werediluted to nanoparaticle concentrations of 25 ppm. The pH of thealiquots was adjusted to 3.5 with ascorbic acid.

Six different laminates were tested: NP-175, 370 HR, TUC-752, SY-1141,SY-1000-2, and FR-408. NP-175 was obtained from Nanya, 370 HR and FR-408were obtained from Isola, TUC-752 was obtained from Taiwan UnionTechnology Corporation and SY-1141 and SY-1000 were obtained fromShengyi. The T_(g) values ranged from 140° C. to 180° C. Each laminatewas 5 cm×12 cm. A surface of each laminate was treated as follows:

-   -   1. Each laminate was immersed into a solvent swell which        included ethylene glycol dimethyl ether and water at a volume to        volume ratio of 1:2 for 7 minutes at 80° C.;    -   2. Each laminate was then removed from the solvent swell and        rinsed with cold tap water for 4 minutes;    -   3. Each laminate was then treated with a permanganate aqueous        solution which included 1% potassium permanganate at a pH above        10 at 80° C. for 10 minutes;    -   4. Each laminate was then rinsed for 4 minutes in cold tap        water;    -   5. The laminates were then treated with a neutralizer solution        of 3 wt % peroxide and 3 wt % sulfuric acid for 2 minutes at        room temperature;    -   6. Each laminate was then rinsed with cold tap water for 4        minutes;    -   7. Each laminate was then immersed in an aqueous bath containing        3% CIRCUPOSIT CONDITIONER™ 231 aqueous acid conditioner for 5        minutes at 40° C.;    -   8. Each laminate was then rinsed with cold tap water for 4        minutes;    -   9. MICROETCH™ 748 solution was then applied to each laminate for        2 minutes at room temperature to micro-etch the laminates;    -   10. The laminates were rinsed with cold tap water for 4 minutes;    -   11. The laminates were then primed for 6 minutes at 40° C. with        one or the aliquots of carboxymethyl cellulose/palladium        catalyst prepared above;    -   12. The laminates were then rinsed with cold water for 5        minutes;    -   13. The laminates were then immersed in CIRCUPOSIT™ 880        electroless copper plating bath at 40° C. and at a pH of 13 and        copper was deposited on the substrates for 18 minutes;    -   14. The copper plated laminates were then rinsed with cold water        for 2 minutes;    -   15. Each copper plated laminate was then placed into a        conventional convection oven and dried for 20 minutes at 105°        C.;    -   16. After drying each copper plated laminate was placed in a        conventional laboratory dessicator for 20 minutes or until it        cooled to room temperature; and    -   17. Each copper laminate was then tested for adhesion using the        conventional Scotch tape test method.

All of the plated copper laminates passed the Scotch tape test. Therewas no observable copper metal stuck to the Scotch tape after removal ofthe tape from the copper laminates.

EXAMPLE 2

A 25 ppm aqueous carboxymethyl cellulose/palladium catalyst was preparedas described in Example 1. The pH of this catalyst solution was adjustedto pH 3.5 with ascorbic acid. The average particle size of the palladiummetal was determined to be 5 nm. The six types of laminates describedabove each with a plurality of through-holes were provided. Thethrough-holes were made conductive by the same process steps andparameters as described in Example 1 for the surface treatment of thelaminates. After the catalyst was applied the through-holes wereelectrolessly plated with the same electroless copper bath described inExample 1.

After plating each board was sectioned laterally to expose the copperplated walls of the through-holes. Multiple lateral sections 1 mm thickwere taken from the walls of the sectioned through-holes of each boardto determine the through-hole wall coverage for the boards. The EuropeanBacklight Grading Scale was used. The 1 mm sections from each board wereplaced under a conventional optical microscope of 50× magnification. Thequality of the copper deposits was determined by the amount of lightthat was observed under the microscope. If no light was observed thesection was completely black and was rated a 5 on the backlight scaleindicating complete copper coverage of the through-hole. If light passedthrough the entire section without any dark areas, this indicated thatthere was very little to no copper metal deposition on the wall and thesection was rated 0. If sections had some dark regions as wells as lightregions, they were rated between 0 and 5. The backlight ratings for the25 ppm carboxymethyl cellulose/palladium catalyst on the six laminatesranged from 4.5 and higher on the 5 scale, which indicates that thecatalyst is generally acceptable for commercial use by industrystandards.

1-4. (canceled)
 5. A method comprising: a) providing a substrate; b)applying an aqueous catalyst solution to the substrate, the aqueouscatalyst solution comprises one or more antioxidants, nanoparticles ofpalladium metal and one or more compounds chosen from polymers having aformula:

wherein R₁, R₂, R₃, R₄, R₅, R₆ and R₇ are the same or different andchosen from —H, —CH₂COOX, —C(O)—CH₃, —C(O)—(CH₂)_(z)—CH₃ and

wherein n is an integer of at least 2, z is an integer of at least 1 andX is —H or a counter cation, and a reaction product of a polymer havinga formula:

wherein R₉, R₁₀, R₁₁, R₁₂, R₁₃, R₁₄, and R₁₅ are the same or differentand are chosen from —H, —CH₃, —CH₂CH₃, —CH₂OH, —[CH₂CHR₈]_(x)—OH,—CH₂CH(OH)CH₃, —(CH₂CHR₈O)_(y)—H, with the proviso that at least one ofR₉, R₁₀, R₁₁, R₁₂, R₁₃, R₁₄, and R₁₅ is —CH₂OH, —[CH₂CHR₈]_(x)—OH,—CH₂CH(OH)CH₃ or —(CH₂CHR₈O)_(y)—H, wherein R₈ is —H or —CH₃, x and yare an integer of at least 1 and n and z are as described above, and aquaternary compound having a formula:

wherein m is an integer from 1 to 16, Y is halogen, Z⁻ is a counteranion, R₁₆, R₁₇ and R₁₈ are the same or different and are —H, —CH₃ or—(CH₂)_(p)—CH₃, and R₁₉ is —H or —CH₃ and p is an integer of 1 to 9, andone or more cross-linking agents, the aqueous catalyst solution is freeof tin; and c) electrolessly depositing metal onto the substrate usingan electroless metal plating bath.
 6. The method of claim 5, wherein thesubstrate comprises a plurality of through-holes.
 7. The method of claim5, wherein the electroless metal plating bath is chosen from a copper,copper alloy, nickel and nickel alloy bath.
 8. The method of claim 5,wherein at least one of R₂, R₃, R₅, R₆ and R₇ is —CH₂COOX or —C(O)—CH₃.9. The method of claim 5, wherein the cross-linking agent is chosen fromone or more of haloepoxy compounds and di-epoxy compounds.
 10. Themethod of claim 5, wherein the nanoparticles are 1 nm to 1000 nm.